Nano- or submicron-sized hierarchic structures such as hollow spheres and capsules have been attracting increasing attention, owing to their wide applications in drug delivery, low dielectric constant substrates, chemical and biological sensing and labeling, optoelectronics, catalysis, wave scattering, lasing, and photonics. Controlled release of heat sensitive drugs such as enzyme, vaccine, peptide, gene, and oligonucleotide from the nanocapsules is important for treatment of cancers and infections due to the improved therapeutic index. Intricate interfacial polymerization or polyelectrolyte layer-by-layer nanoassembly has been widely applied to fabricate protective nanocapsule shells capable of preventing oxidation/degradation of the encapsulated agents. Fast disintegration and easy degradation of the nanocapsule shells after oral or parenteral administration and the difficulty with maintaining the stability of nanocapsules suspensions impede its medical applications and industrial development. Ceramic materials show promise because they are less toxic, possess good thermal and chemical stability, and are biocompatible. Interfacial transport and phase separation play a role in the nanofabrication, and good control of the nanostructure needs an optimization of generation conditions based on proper analysis and design of the multiple-phase chemical engineering problem. Electro-hydrodynamic force has been applied to generate steady coaxial jets of immiscible liquids and fabricate nanocapsules of aqueous-based ingredient. Hollow spherical indium and zinc sulfide have been synthesized via a template-free solvothermal route at high temperature. The so-called hollow sphere structures are composed of many aggregated particles, far from perfect for desired shape and sizes. Fabrication via vapor-solid interfacial reaction such as laser ablation, molecular beam epitaxy, and chemical vapor deposition requires long process time, and high vacuum and temperature. The shells of nano hollow spheres of tailored dimensions and compositions for Au, Ag, CdS, ZnS, silica, or titania can be self-assembled in the presence of surfactants or sacrificial templates such as preformed rigid inorganic nanoparticles or in-situ polymerized cores such as carbon spheres, polystyrene latex beads, silica colloids, or block copolymer vesicles. The sacrificial templates or surfactants, however, have to be removed by time consuming high temperature calcination or solvent extraction, and most of times the removal leaves behind cracks, defects, or carbonaceous impurities with the hollow nanospheres. Furthermore, the engaged toxicity/pollution by the surfactants and solvents to the drug or target biological medium precludes their usage in the applications.
Aerosol methods are promising in that droplet and particle size, size homogeneity, evaporation rate, vapor-liquid interfacial transport, and reaction kinetics can be well controlled, and the process can be easily scaled up. Aerosol assisted evaporation induced self assembly (EISA) has been successfully applied to make ordered core shell nano structures Although metal oxide nano hollow spherical particles can be formed by thermal decomposition and succeeding surface gelation via aerosol pyrolysis, the method can not be applied to fabricate hollow spherical titania or germania from their highly reactive precursors without templating and/or controlling the interfacial diffusion of reactant molecules. To encapsulate heat sensitive biological materials the fabrication temperature is limited.
Accordingly, there is a need for developing a general, low-temperature, low cost, template free, nondestructive fabrication method for the metal oxide nanostructures.